2 Major CA Faults Could Produce Back-To-Back Earthquake Disasters, Study Says

In 1700, two earthquakes that likely erupted within hours along two different fault zones in California may have unleashed a rare, double-barreled disaster, researchers believe.

According to a new study published in Geosphere, it wouldn't be impossible for history to repeat itself.

The research revisits the massive 1700 Cascadia quake — known for sinking coastal villages and sending a “ghost tsunami” across the Pacific to Japan — and examines whether a corresponding rupture occurred on the northern San Andreas Fault.

While the evidence remains inconclusive, sediment records show that many past earthquakes along the two systems happened within decades — or possibly hours — of one another.

The Pacific coast of the United States sits atop a complex network of tectonic boundaries. North of Cape Mendocino, the Juan de Fuca Plate dives beneath North America along the Cascadia Subduction Zone, while to the south, the Pacific and North American plates grind past each other along the San Andreas Fault. Scientists say the possibility that both could rupture in quick succession would redefine earthquake risk across the West Coast.

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“It’s kind of hard to exaggerate what a M9 earthquake would be like in the Pacific Northwest,” Dr. Chris Goldfinger, a paleoseismologist at Oregon State University and lead author of the new study said. “And so the possibility that a San Andreas earthquake would follow, it’s movie territory.”

Goldfinger’s team at Oregon State University found that over the past 3,100 years, at least ten Cascadia quakes coincided closely with San Andreas events, leaving distinctive “doublet” layers in offshore sediment. The findings suggest that California’s two largest fault systems may sometimes rupture in sequence — a phenomenon scientists call partial synchronization.

“The double impact of the two earthquakes along the Cascadia and San Andreas faults was probably not just a black-swan, chance-in-a-million," Goldfinger told the Los Angeles Times.

“This is, like, most of the time. The only exception in the last 2,500 years was 1906 — that was the only event” in which a major quake on the northern San Andreas fault wasn’t preceded by a massive Cascadia rupture, according to the analysis.

That means what happened in 1700 may not have been unique at all. The team’s data indicate that similar back-to-back earthquakes likely occurred between 1425 and 1475, 1175 and 1225, and around A.D. 825 and 475 B.C., suggesting a recurring pattern of linked seismic activity.

The Geological Society of America said the discovery was made partly by accident, after researchers collecting sediment off the Pacific Northwest drifted about 90 kilometers too far south during a 1999 expedition, into San Andreas territory.

“When I woke up, I was pretty hot,” Goldfinger said in the GSA release. “But, once we were there, I thought, ‘well, let’s take a core here.’”

Inside that core, scientists found a peculiar pattern — layers of sediment arranged in pairs, or “doublets,” showing evidence of near-simultaneous shaking along both fault systems.

“There were these big, thick, sandy doublet events where it had a fine-grained element, and on top of it was a very coarse-grained sandy unit. And we were just scratching our heads,” Goldfinger said.

In some cases, the sediment suggests the second quake struck within minutes or hours of the first, meaning much of the U.S. Pacific coast could experience violent shaking nearly simultaneously.

“I’m from the Bay Area originally,” Goldfinger said. “If I were in my hometown of Palo Alto, and Cascadia went off, I think I would drive east. There looks to me like a very high risk the San Andreas would go off next.”

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